Slowed crossbridge cycling constitutes a fundamental defect in failing myocardium. Though functionally deleterious, it is associated with more economical and efficient energy utilization. The mechanism of slowed crossbridge cycling is unclear. Data in humans implicate a non-myosin isoform related alteration(s) of the myofilament as a major contributor. To study this problem, we have employed the Dahl salt sensitive (DS) rat as a model of compensated cardiac hypertrophy and the transition to systolic failure (SF). When fed high salt diet, DS rats develop combined pressure and volume overload. Previously documented left ventricular (LV) and myocardial abnormalities ascribable to slowed crossbridge cycling in DS rats become manifest during the transition to SF and are very similar to those observed in human myocardium. We have recently documented an in vitro thin filament velocity defect in DS rats with SF that may be an important mechanism of slowed crossbridge cycling. Additional data suggest that protein kinase C (PKC) beta activation resulting in altered troponin (Tn) I and/or TnT phosphorylation, possibly mediated by endothelin-1 (ET-1) signaling, underlies this thin filament defect. In Aim I, we will use a PKC-beta specific inhibitor to "normalize" myofilament behavior in DS rats with SF and delineate the myofilament effects of chronic ET-1 blockade. Studies will be performed in the isolated LV and in skinned strips and isolated native thin filaments (NTFs) (in vitro motility and force assays). Correspondingly, we will treat control NTFs with PKC-beta to determine if this simulates abnormalities found in SF. In Aim II, we will delineate myofilament abnormalities and their mechanism(s) in a model of diastolic heart failure (DHF) in the DS rat. Preliminary data suggest that myofilament abnormalities are present in DHF that are similar to SF, but compensated by increased Ca2+ pumping. As part of this aim, we will measure Ca2+ transients in isolated myocytes to determine the functional role of Ca2+ pumping in the transition to SF. In Aim III, we propose to develop a mass spectroscopy-based approach to assess site-specific phosphorylation of Tnl and TnT in compensated and failing DS rats. This approach, combined with Aims I and II, should provide novel information about the details of thin filament phosphorylation in failing myocardium and may suggest new therapeutic targets.